Head CC's for Compression Calculation
- Thread starter ursus
- Start date
True, but what is the cylinder head volume of a 5.5/1 or a 6.5/1 head? That is the question.
It’s not that simple. Your closest comparison is to acquire a new 5.5:1 and a 6.5:1 head to measure the volume.
Compression ratio will vary from engine to engine simply using a chamber cc amount. Honestly, your measurement looks high, resulting in a low compression ratio, certainly outside of what you’re asking.
Thodge
Senior Member
Snyder’s list their 6:1 head as having a 152cc chamber. Knowing that and the displacement of the stock head should let you calculate others. There’s a lot more to calculating your actual ratio. Especially on a side valve engine like these where there’s a lot more room in the head beside the cylinder. Whether or not the cylinders have been bored affects the ratio as well. The only way to know for sure is to fill the cylinder up with fluid to the spark plug hole while the piston is at TDC then extract and measure it.
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It is not difficult to measure head volume. Lay the head on your work bench upside down. Clean the surface and the chambers. Remove and then replace the spark plugs finger tight, then just a little more. Get a piece of plexiglass 1/4" thick or thicker. Lay it on top of the head over one of the chambers and drill a small hole --maybe 1/4" in the Plexiglas near the top of the combustion chamber. Use a 1/2" bit to open the top of the hole into a bit of a funnel shape. Clean up everything and place the plexiglass back on the head--use a thing coat of grease or Vaseline to seal the Plexiglas to the head. You will need a syringe--no needle--that is calibrated in CC's. Fill the syringe to a know mark with ATF and inject that ATF into the combustion chamber. Remove any air bubbles. Keep track of how much ATF it takes to completely fill the combustion chamber. Rinse and repeat --hopefully, you will get the same answer. Then move to another chamber. I drain out the ATF by removing the spark plug and using a funnel to save the ATF--or you could just flood the bench...or floor.
Its really a simple--but tedious--process.
Its really a simple--but tedious--process.
Thodge
Senior Member
So since I was already doing math this morning, I figured I’d do ALL the math. These numbers are assuming stock bore and stroke giving 50.12 ci cylinder and would include any extra space for deck height and gasket thickness.
Stock 4.22:1 head: 194.5 cc
Police 5.22:1 head: 157.3 cc
5.5:1 head: 149.3 cc
6.0:1 head: 136.8 cc
Stock 4.22:1 head: 194.5 cc
Police 5.22:1 head: 157.3 cc
5.5:1 head: 149.3 cc
6.0:1 head: 136.8 cc
On the model A engine measuring piston protrusion when setting up for a high compression head.The space between the piston crown and head at TDC is called quench, and ideally it’s .040.. Fly cutting is relieving the head above the piston, that dimension is critical for factoring quench.
Measuring combustion chambers is interesting, but don’t be worried about a few cc deviation, the engine will run fine. It is important not to have too much compression, a combustion chamber too small effects valve ‘breathing’
Measuring combustion chambers is interesting, but don’t be worried about a few cc deviation, the engine will run fine. It is important not to have too much compression, a combustion chamber too small effects valve ‘breathing’
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johnneilson
Well-known member
rotorwrench
Well-known member
A person needs the volume of the cylinder first. If it is .060" oversize versus standard bore then the ratio will be different with the same clearance volume. The ratio is the difference between the two. Just make sure to use the equivalent volume figures, ie CCs or cubic inches for both volumes.
Thank you, johnneilson! This is what I was looking for. I have a couple of heads that cc'd out at 167 and 149 cc and was curious about their compression ratios when mounted on an engine. The heads are of unknown provenance as somebody had ground off any identifying details, probably in an effort to simulate a stock appearance.
johnneilson
Well-known member
Rotor brings up a great point about bore size and influence on CR
There are other factors such as piston protrusion up into chamber or gasket volume
Valves can also have some influence
That spreadsheet I made has been pretty darn accurate in that it tracks very closely when I do a final assembly and run the whistler on it
J
There are other factors such as piston protrusion up into chamber or gasket volume
Valves can also have some influence
That spreadsheet I made has been pretty darn accurate in that it tracks very closely when I do a final assembly and run the whistler on it
J
nkaminar
Well-known member
You can measure the combustion chamber volume with the head mounted on the engine. That takes into account the gasket and any oversize of the bore. Take the plugs out and hand crank the engine so that the #1 piston is at top dead center on the compression stroke. The pin in the dimple should be accurate but you can check with a wire or screwdriver in the spark plug hole that is resting on the top of the piston.
Use the syringe and ATF talked about in post #7 and fill the combustion chamber to the bottom of the spark plug hole. Use that volume to calculate the compression ratio. Turn the crankshaft 180 degrees and you can measure #2. Use the wire or screwdriver to make sure the piston is at the top. Repeat for #4 then #3. When you first start the engine there will be lots of oil smoke.
There is more than compression ratio in head design. The head must allow good breathing and turbulence for proper combustion. The head shown in post #10 will provide good breathing.
Use the syringe and ATF talked about in post #7 and fill the combustion chamber to the bottom of the spark plug hole. Use that volume to calculate the compression ratio. Turn the crankshaft 180 degrees and you can measure #2. Use the wire or screwdriver to make sure the piston is at the top. Repeat for #4 then #3. When you first start the engine there will be lots of oil smoke.
There is more than compression ratio in head design. The head must allow good breathing and turbulence for proper combustion. The head shown in post #10 will provide good breathing.
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